Printed circuit board (PCB) manufacturing processes typically comprise many steps, in part because of the increasing demand for enhanced performance. Surface circuits on PCBs usually include copper and copper alloy materials that are coated to provide good mechanical and electrical connection with other devices in the assembly. In the production of a printed circuit board, a first stage comprises preparing the circuit board and the second stage comprises mounting various components on the circuit board.
There are generally two types of components that are attachable to the circuit board: a) legged components, such as resistors, transistors, etc., which are attached to the circuit board by passing each of the legs through a hole in the board and then ensuring that the hole around the leg is filled with solder; and b) surface mount devices, which are attached to the surface of the board by soldering with a flat contact area or by adhesion using an adhesive.
Plated through hole printed circuit boards may be fabricated by a process comprising the following sequence of steps, although other sequences of steps may also be used. Fresh water rinses may be interposed between each step.    1) Drill holes through copper clad laminate;    2) Process boards through standard plated through hole cycle to plate electroless copper in the holes and on the surface;    3) Apply a plating mask;    4) Electrolytically plate copper to desired thickness in the holes and on the exposed circuitry;    5) Electrolytically plate tin in holes and on exposed circuitry to serve as an etch resist;    6) Strip the plating resist;    7) Etch the exposed copper (i.e., copper not plated with tin);    8) Strip the tin;    9) Apply, image and develop a soldermask such that the soldermask covers the substantially entire board surface except for the areas of connection;    10) Protective layer; and    11) Clean and microetch the Areas of Connection.
Other examples of sequences of steps that may be used to prepare the printed circuit boards in the first stage are described in U.S. Pat. No. 6,319,543 to Soutar et al., U.S. Pat. No. 6,656,370 to Toscano et al., and U.S. Pat. No. 6,815,126 to Fey et al., the subject matter of each of which is herein incorporated by reference in its entirety.
Solder masking is an operation in which the entire area of a printed circuit board, except solder pads, surface mount pads, and printed through-holes, is selectively covered with an organic polymer coating. The polymer coating acts like a dam around the pads to prevent the undesirable flow of solder during assembly and also improves the electrical insulation resistance between conductors and provides protection from the environment.
The solder mask compound is typically an epoxy resin that is compatible with the substrate. The solder mask may be screen printed onto the printed circuit board in the desired pattern or may also be a dry film photoimageable solder mask that is coated onto the surface. Both types of solder masks are generally well known to those skilled in the art.
The contact areas include wire-bonding areas, chip attach areas, soldering areas and other contact areas. For example, contact finishes must provide good solderability, good wire bonding performance and high corrosion resistance. Some contact finishes must also provide high conductivity, high wear resistance, and high corrosion resistances. One typical prior art contact finish coating may include an electrolytic nickel coating with an electrolytic gold layer on top, although other coatings are also known to those skilled in the art.
Soldering is generally used for making mechanical, electromechanical, or electronic connections to a variety of articles. In the manufacture of electronic equipment utilizing printed circuits, connections of electronic components to the printed circuits are made by soldering of the leads of the components to the through-holes, surrounding pads, lands and other points of connection (collectively, “Areas of Connection”). Typically the connection occurs by wave soldering techniques.
To facilitate this soldering operation, the printed circuit fabricator is required to arrange that the through-holes, pads, lands and other points of connection are receptive to the subsequent soldering processes. Thus these surfaces must be readily wettable by the solder and permit an integral conductive connection with the leads or surfaces of the electronic components. Because of these needs, printed circuit fabricators have devised various methods of preserving and enhancing the solderability of surfaces. Examples of such methods are described in U.S. Pat. No. 6,773,757 to Redline et al. and in U.S. Pat. No. 5,955,640 to Ferrier et al., the subject matter of each of which is herein incorporated by reference in its entirety.
As discussed in the U.S. Pat. Nos. 6,773,757 and the 5,955,640 (incorporated herein by reference), it is known that immersion silver deposits provide excellent solderability preservatives, which are particularly useful in the fabrication of printed circuit boards. Immersion plating is a process which results from a replacement reaction whereby the surface being plated dissolves into solution and at the same time the metal being plated deposits from the plating solution onto the surface. The immersion plating initiates without prior activation of the surfaces. The metal to be plated is generally more noble than the surface metal. Thus immersion plating is usually significantly easier to control and significantly more cost effective than electroless plating, which requires sophisticated auto catalytic plating solutions and processes for activation of the surfaces prior to plating.
However, the use of immersion silver deposits can be problematic because of the possibility of solder mask interface attack (SMIA) wherein galvanic attack may erode the copper trace at the interface between the solder mask and the copper trace. SMIA is also known by other names such as solder mask crevice corrosion and simply galvanic attack at the solder mask interface. Regardless of the name, the problem comprises a galvanic attack at the solder mask-copper interface. Therefore, there is a need for an improved immersion plating process that can minimize or eliminate the interfacial galvanic attack. To that end, the inventors of the present invention have discovered that the use of ultrasonics in combination with an immersion plating process, particularly an immersion silver plating process, can provide a beneficial result. This interfacial galvanic attack arises as a result of the soldermask-copper interfacial structure and the immersion plating mechanism.
Ultrasonics have been used in cleaning printed circuit boards prior to plating. Ultrasonics have also been used to aid in filling microvias and blind microvias. For example, U.S. Pat. No. 5,705,230 to Matanabe et al., the subject matter of which is herein incorporated by reference in its entirety, describes a method for filling small holes or covering small recesses in the surfaces of substrates, wherein during deposition, a controlled varying voltage and/or an energy such as low frequency, high frequency or ultrasonic vibrations is applied to the substrate in order to improve the efficiency of the plated deposit. Likewise, U.S. Pat. No. 6,746,590 to Zhang et al., the subject matter of which is herein incorporated by reference in its entirety, describes the use of ultrasonic energy to enhance plating processes. However, ultrasonics have not been used in combination with immersion plating processes in the manner described herein.